It is useful to be able to transfer light energy traveling in an optical fiber from one waveguide to another dissimilar waveguide, i.e., one with different propagation characteristics. In fiber optics, a single birefringent fiber can be considered to be two dissiimilar waveguides in that the fiber can guide light and maintain its polarization in either of two independent polarization modes. To be able to control the power transfer between these two modes is highly desirable, and, if controlled in response to a varying signal results in the modulation of the power transfer.
In copending application Ser. No. 556,305, filed on Nov. 30, 1983, entitled "BIREFRINGENT FIBER NARROWBAND POLARIZATION COUPLER," and assigned to the assignee of the instant application, a four port directional coupler apparatus for controlling the transfer of power between dissimilar, linear, low loss waveguides and a method for causing such transfer was disclosed. The same coupler apparatus was also described in "Birefringent-fiber polarization coupler, "R. C. Youngquist, et al. OPTICS LETTERS, Vol. 8, No. 12, December 1983, pages 656-658. The dissimilar waveguides are the two orthogonal polarization modes of a single highly birefringent monomode optical fiber waveguide which is capable of maintaining the polarization of light traveling therein over long distances.
As disclosed in that application and paper, a plastic block with ridges machined thereon and a polished, flat surface are used to exert mechanical stress on a birefringent optical fiber waveguide. The ridges are one-half beat length wide (or an odd multiple thereof) and are separated by one-half beat length (or an odd multiple thereof). The beat length is equal to the distance traveled along the waveguide between locations at which the light traveling in the two orthogonal polarization modes are in phase. The purpose of the stress created by such ridges is to abruptly change the orientation of the birefringent axes of the fiber at periodic locations separated by one-half beat lengths.
Optical power can be transferred by pressing the ridges against the fiber which is resting on the polished, flat surface. The fiber is oriented so that the principal axes of birefringence are at some appreciable angle to the direction of the principal stress vector, preferably 45.degree.. The stress created by the ridges causes the orientation of the axes of birefringence to shift abruptly at the edges of each of the ridge surfaces. The periodic abrupt shifting of the birefringent axes for an odd multiple of one-half beat length and then reversion of the axes to their original states for an odd multiple of one-half beat length by the absence of stress can cause substantially all the power launched in one polarization mode at the input of the device to be transferred to orthogonal polarization mode at the output of the device.
The device thus described can be used as an amplitude modulator by applying a time-varying signal to the ridges to cause the stress to vary periodically in time as well as in distance. However, the time-varying stresses must be applied uniformly at all of the stressed regions of the fiber. Furthermore, the signal must be of sufficient amplitude to move the significant mass of the block. Thus, a need exists for a device which will apply a time varying stress to a birefringent fiber without requiring the movement of a significant mass of material.